1. Field of the Invention
This invention relates to mechanical seals, which are devices used for preventing fluid leakage from pumps, mixing vessels, gear boxes and other pieces of equipment where fluid is situated between stationary and rotating parts.
2. Background to the Invention
A mechanical seal is normally located in a seal cavity which is situated, for instance, where a rotating shaft enters the stationary part of the equipment. It typically comprises two parts, one termed the "rotary" which is fixed to the rotating shaft, and the other termed the "stationary" which is fixed to the body of the equipment.
Typically, mechanical seals consist of a number of components: as a result, it is difficult and time consuming to instal them properly. Problems are encountered in setting the seals to their correct working lengths and in protecting the seal components (to avoid soiling and damage) during assembly. As a result, so-called cartridge-mounted mechanical seals, which are factory-assembled to form pre-set units and do not require installation measurments to be taken on fitting, have become increasingly popular.
Typically, cartridge-mounted mechanical seals include a stainless steel sleeve which forms an inner cylindrical member which, in use, surrounds the rotating shaft. At one end of the sleeve, there is provided a groove to accommodate an `O` ring between the sleeve and the shaft.
A cartridge-mounted mechanical seal has rotary and stationary components which contact one another at their respective, adjacent, faces. There is a need to maintain good mating contact between the two faces to ensure efficient operation of the seal, and thus often some form of biassing means is incorporated in the seal, either a rotary biassing means to urge the rotary component face into mating contact with the stationary component face, or a stationary biassing means, acting against the stationary face to urge it into contact with the rotary face.
The biassing means may, for example, take the form of a spring or springs, usually located in one or more longitudinal bores provided between the end of the seal sleeve and the rotary or stationary component of the seal, which springs act against the rotary or stationary component respectively to urge it in a direction towards the other component.
Alternatively, bellows are sometimes used as a biassing means. These are typically made from a resilient metal, and usually supplied in the form of a bellows unit for securing to a seal sleeve during assembly of the seal. Apart from helping to maintain seal face contact in the seal, the bellows also effectively function as a secondary seal, in addition to the seal faces themselves against which they act, removing the need for additional secondary sealing elements such as `O`-rings.
One problem which can arise in the use of mechanical seals is that out-of-alignment between parts of the equipment in which a seal is used can disrupt contact between the stationary and rotary component faces.
Most mechanical seals are dimensionally suitable for up-grading pumps with packed glands into effectively sealed units, without equipment modifications. However, most seal designs do not account for the fact that pumps were often originally designed for use with compression-type gland packings. The stuffing box face of a packed gland pump does not have to be perpendicular to the shaft centre line for the compression packing to work. Thus, in an effort to make savings in production costs, some pump manufacturers do not finish machine the stuffing box face, with the result that the face cannot be square to the shaft. Even on rotating equipment which is designed to take mechanical seals, a lesser degree of out-of-squareness can still exist due to component manufacturing and assembly tolerances.
When a seal is fitted to a pump without this out-of-squareness being rectified, then angular mis-alignment will exist between the rotary and stationary seal components.
Where angular mis-alignment exists, the robustness of the rotary seal floating member/parts determines whether the rotary unit of the seal aligns to the rotating equipment shaft or to the stationary seal component. If the rotary component aligns to the shaft, then a gap will open up between the rotary and stationary seal faces due to the angular mis-alignment. Alternatively, the rotary component can align to the stationary seal face, which causes fretting underneath the dynamic `0` ring (between the rotary component and the seal sleeve on which it is mounted) due to the `0` ring moving against the seal sleeve. It also causes fatigue of springs acting as rotary biassing means, due to changes in the spring operating length which are required to keep the rotary and stationary component faces in contact with each other. Very often, the results of angular mis-alignment are a mixture of these two situations, which combine to considerably shorten the seal life.
In rotary metal bellows seals, where a bellows unit acts as biassing means against the rotary seal component, fatigue is a particular problem if the metal bellows are of a welded construction. Welded structures are particularly prone to cracking when fatigued; in the case of welded metal bellows this means that irreparable damage is caused to the seal unit.
A so-called self-aligning stationary is thus incorporated in many seals to compensate for the angular mis-alignment which can exist between equipment shafts and bodies. A self-aligning stationary is a stationary component which is pivotally mounted in a seal gland plate so as to allow a degree of pivotal movement, when the seal is in use, of the stationary component relative to the stationary parts of an item of equipment in which the seal is used, about an axis perpendicular to the longitudinal axis of the seal, and hence maintain the stationary and rotary components of the seal substantially in alignment despite any out-of-squareness in the equipment.
Such a "self-aligning stationary" is capable of aligning to the rotary component of the seal, thus compensating for any out-of-squareness (i.e. between rotating and stationary parts) in the equipment in which the seal is used. Whilst the rotary component of the seal, in use, rotates with the rotating parts of the equipment, the stationary component of the seal is not completely fixed relative to the stationary parts of the equipment, and can thus remain aligned to the rotary component of the seal (maintaining contact between the stationary and rotary seal faces) whatever the relative positions of rotating and stationary parts of the equipment.
The stationary component (seal face) will typically be mounted about one or more primary pivot pins set in a pivot ring which is in turn mounted about secondary pivot pins set in an integral gland plate included in the seal, or in another part of the seal which, in use, remains fixed in position relative to stationary parts of the equipment.
Various self-aligning stationarys of the type referred to are already known, one in particular (although not one necessarily suitable for use with all types of seal faces) having been described in our European Patent No. EP-0,098,747B.
The use of biasing means such as springs or bellows, and the alternative of a self-aligning stationary as a means of ensuring constant contact between rotary and stationary component faces, is often not sufficient on its own to maintain the contact needed. It has often proved impossible to compensate completely for out-of-squareness arising between the two components in use.